Radar System Engineering

308 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 917
is a misalignment between the beam axis and horizontal, the synchros in
the gyro-torque unit transmit the error to the amplifier, a phase-sensitive
detector whose output controls the servomotor, which in turn causes the
linkage system to tilt the antenna up or down until the beam axis is again
horizontal. Because the gyro is mounted on the scanner, it is liable to
the precession caused by the inertia of the supporting gimbal system.
Under some conditions this precession causes the gyro to indicate a false
vertical, thus producing serious errors in the stabilization system.
Although in the above example the gyro is mounted on the rotating
yoke, the effects of precession are avoided in the most recent designs
by mounting the gyro on the airframe and providing a take-off on each
gimbal axis. A voltage indicating the correct tilt angle is fed to the
servoamplifier. A special rotary inductor mounted on the scanner base
provides this voltage, which it computes as a function of the antenna
relative azimuth and the roll and pitch voltages from the gyro.
As previously discussed, the purpose of antenna stabilization is to
preserve range performance by assuring constant illumination in nonlevel
flight as in level flight. In considering the accuracy requirements
various factors must be compared, such as beam shape and operational
use contemplated for the radar. If a pencil beam moves up or down
because of maneuvers of the aircraft or poor stabilization, the scope
signal will fade. Quantitatively, if a beam moves up or down by one-half
its beamwidth, it can be shown that the set will suffer a range reduction
of 29 per cent. The range will be reduced 10 per cent if the beam varies
from the stabilized position by 0.28 beamwidth and by 5 per cent if the
beam varies 0.19 beamwidth. As a range reduction of 10 per cent is
considered the maximum allowable, it can be seen that the beam must be
stabilized so that it remains within 0.28 beamwidt h of a completely
stabilized position. The adequacy of the above calculations has been
verified by rough observations in flight. Beam stabilization in current
airborne navigation radars should be accurate within about t 1.3°,
depending on the set, lest range performance suffer noticeably.
In practice it is quite difficult to realize stabilization accuracies high
enough to satisfy the foregoing tolerances for the various types of beams
and radars. This is principally due to the early stage of airborne stabilization
development and the stringent weight requirements on airborne
gyroscopes. Current developments should lead to more highly accurate
lightweight airborne antenna stabilization. The present static accuracy
of airborne gyros ranges from + # to + #. However, errors of from
1° to 5° may occur if the erection mechanism is not cut off in a turn.
This error is a function of the duration of the turn.
Shipborne Antenna Stabilization.—In recent practice the stabilization
of airborne ground-mapping antennas is by the line-of-sight method, in